Cortical Processing and Organization in the Hearing Subject
Functional specialization is a common characteristic of the cerebral cortex. Globally, regions are specialized to perform particular sensory or motor functions. Within extrastriate visual cortex of humans, monkeys and cats, areas have been identified that are further specialized for spatial, motion, and pattern processing. The behavioural correlate for such functional specializations, or a “division of labor”, within auditory cortex is largely unknown. The long-term goal of this work is to elucidate the behavioural “division of labor” within auditory cortex and determine the relative contributions that the different auditory fields make to acoustic behaviours. These results, when combined with investigations of underlying cerebral connections, will provide evidence for, or refute, which hierarchical or network theories best explain processing in auditory cortex. We are combining a battery of psychophysical tests, reversible deactivation, pathway tracing, and activity-dependent measures to dissect the functional plan of auditory cortex and the specific cerebral regions mediating audiomotor responses controlled by the superior colliculus.
We are examining the thirteen commonly recognized regions of acoustically-responsive cortex: the four tonotopic fields (AI, AAF, PAF & VPAF) and nine non-tonotopic areas (AII, AES, DZ, dPE, iPE, vPE, VAF, IN, & T). Two general classes of tasks are being used to determine if areas specialized for “spatial and motion” and “pattern and temporal” processing can be identified.
I) Spatial & Motion Processing. Reversible cooling deactivation is being used determine the areas of auditory cortex responsible for the accurate reorienting of the head to an acoustic stimulus and the discrimination of stationary and moving sounds. Tasks require the subject to: 1) reorient its head to an acoustic stimulus; 2) accurately localize a stationary acoustic stimulus without an audiomotor response; and 3) accurately discriminate the direction of a moving acoustic stimulus.
II) Pattern & Temporal Processing. Reversible cooling deactivation is being used to determine the auditory cortex areas mediating pattern and temporal processing. Tasks require the subject to: 1) discriminate temporal patterns of the same duration; 2) discriminate between acoustic stimuli that differ only in their temporal duration; and 3) discriminate between different natural vocalizations.
After behavioural training, pairs of cooling loops are bilaterally implanted over discrete areas of auditory cortex. These loops will both be bilaterally and unilaterally deactivated, and any resulting impairments identified. At the conclusion of testing, 2-deoxy-14C-glucose (2DG) uptake techniques will be used to determine deactivation extent.